Catalytic converters have been widely used in automobile exhausts to the reduce the emission of harmful gases, such as carbon monoxide and hydrocarbons, into the atmosphere. Active catalyst materials placed within the catalytic converter may be used to chemically convert toxic fumes to relatively less hazardous gases. For example, one important chemical reaction that takes place within the catalytic converter is the catalytic oxidation of carbon monoxide (CO) to produce carbon dioxide (CO2). In addition to the reduction of CO emission to address environmental concerns, the mitigation of CO gas can also aid other processes within the catalytic converter that may be hampered in the presence of CO, such as the catalytic combustion of hydrocarbon gases.
While many commercial catalysts exist, such as platinum—rhodium alloys or alumina-supported palladium metal, improvements are still needed. For example, in a typical internal combustion engine more than half of total hydrocarbon emissions (50-90%) occurs during the initial cold starting of the engine (e.g., when the catalyst is below 600 K). Current commercial catalysts are unable to catalytically oxidize CO at such low temperatures. The CO oxidation often proceeds at a measurable rate only after the catalyst has been heated to 600 K by, for example, the exhaust gases, prior to which time a significant portion of hazardous CO-containing fumes may be released into the atmosphere. Furthermore, current CO oxidation catalysts are typically high in cost due to the use of materials such as platinum, rhodium, and palladium in relatively high amount, often in the form of metallic particles having a diameter of about 10 nm.
Gold nanoparticles have been suggested as an alternative to present CO oxidation catalysts. For example, Haruta (Haruta et al., J. Catal. 1989, 115, 301) demonstrated that gold clusters supported on transition metal oxides such as TiO2 perform the catalytic CO oxidation at temperatures of about 200 K. Many other prior investigators have dealt with the catalytic oxidation of CO, yet it is believed that these investigations, including the result of Haruta, are not optimal when compared with that of the present invention.
Accordingly, improved catalysts for CO oxidation are needed.